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Worm infection during and after dry weather or drought in the (normally) moderate to high rainfall regions

By Deb Maxwell, ParaBoss Executive Officer

September 2018


Many livestock producers are surprised to find their stock are dying from worms during a drought, and others that have fed their stock through a drought are dismayed when they succumb to worms soon after it rains.

What actually happens to worms during long dry periods or drought? And how are they best controlled during these times in those moderate to high rainfall regions where worms are normally present?

The key is to conduct regular worm egg counts each 4–6 weeks to take the guesswork out of worm control.

Worms already in the stock

First and foremost, your stock likely carried an existing population of worms into a drought. Worms live for many months, so your stock can easily be suffering from worms they picked up before it became dry.

A WormTest is the number one way to tell how wormy your stock are.

Worm larvae picked up in first 6 months of dry times

When there was previous rain, eggs developed to infective worm larvae that ended up on the pasture and ground. While the worm eggs need moisture to hatch and become larvae, the larvae don’t need moisture to survive.

These larvae are either ingested by the stock or they die when they run out of stored energy. The majority—about 90%—die in about 6 months under cooler conditions, 3 months under hot conditions, and even faster when extremely hot.

A small percentage of larvae live longer than this, especially in cooler situations, with very few surviving over one year.

So despite lack of rain, there can be continued ingestion of worm larvae from dry pastures for many months, and worm burdens in the stock can rise quite fast when they lose condition and their immunity declines.

4–6 weekly mob WormTests are recommended for drought-affected stock in the moderate to high rainfall regions.

Worms picked up after about 6 months of dry times

After the worm larvae that hatched in good times are gone, worms can only originate from new contamination, that is, from the worms living in the stock during drought.

The question is: will these eggs that are landing on a dry paddock survive, develop to infective larvae, and become available for ingestion by the stock?

Generally, all nematode worm species will hatch and develop into larvae within a moist faecal pellet, but the brown stomach worm (Teladorsagia) and black scour worm (Trichostrongylus) are able to survive within the faecal pellet for many weeks during very dry times. Barber’s pole worm (Haemonchus contortus) has less ability to do this and will generally die within a week or so if there has not been about 10–15 mm of rain.

Once sufficient rain dissolves the dung pellet or pat, any living scour worm larvae are released onto the surrounding ground and they can movve up the new pasture and be ingested by stock (see section: Worms after it rains).

Worm pickup from low pasture, when trail feeding grain, or around springs and watercourses

Stock can pick up more worm larvae when they are eating the last of the pasture because larvae are concentrated lower on the pasture plants. The same is true whether it is drought or not, if the pasture is kept grazed quite short.

There is some concern that sheep eat dirt with pulverised dry dung when eating grain trailed on the ground and will gain a worm infection under these conditions. Parasitologist and Paraboss Technical Committee member, Dr Brown Besier says, “We see no significant pick up of these larvae from the ground when the paddocks are dry with no pasture. It’s not until some green pasture has developed that the new larvae that were still in the faecal pellets start to move out, and in practice, significant re-infection of sheep is usually some weeks later, once green pasture is well established.” (See section: Worms after it rains).

Where green grass continues to grow along creekbanks, channels and drains, in soaks from springs, or even where a trough or pipe is continually leaking into a large area, there is potential for eggs to develop to infective larvae. Stock will concentrate on this looking for any available feed, therefore there will also be a concentration of dung dropped there. If the soil is continually moist, the dung will soften and if it contains worm eggs the larvae will develop and emerge. The stock that continue to graze there can then become infected with worms.

Monitor worm egg counts each 4–6 weeks if they have access to moist areas with growing grass.

Worms after it rains

Drought-breaking rain is not required to cause a large increase in worms. Worm eggs develop and are released from the faecal pellets after about 10–15 mm of rain falling across a few days, although in hot areas some follow up rain may be required.

Green pick (new green pasture) resulting after rain is a good indicator that worms are hatching and larvae are becoming available on pasture.

Monitor worm egg counts 4–6 weeks after rainfall that results in green pick, and continue monthly testing as the pasture comes away.

When should I drench?

You should drench when the cost of regular testing, mustering and drenching is a lower cost than the cost of production loss from the worms.

When stock are in lighter condition and when pastures are of lower quality and quantity, stock are not able to mount as effective an immune response and become more susceptible to worms. At these times they should be drenched at lower worm egg counts than when in good condition and on good pasture.

Drench at these worm egg count thresholds:

  • Barber’s pole worm:
    • Stock in poor condition and on poor pasture—600 epg
    • Stock in good condition and on good pasture—1200 epg
  • Scour worms:
    • Stock in poor condition and on poor pasture—200 epg
    • Stock in good condition and on good pasture—250–300 epg

You can also use the WormBoss Drench Decision Guides to assist with your decisions on drenching time.

If you are relying on visible signs to make a drench decision, you are leaving it too long. Consider these three states of “worminess”.

  • A low to moderate worm burden is slight anaemia and/or weight loss that cannot be seen and those effects are not as costly as mustering and drenching. Don’t drench.
  • A high burden is causing more anaemia and/or weight loss, but these will also be invisible or extremely difficult to judge visually, but will now be causing significant economic production losses. The cost of these losses that you cannot see is already outweighing the costs associated with regular testing, mustering and drenching costs. Do a WormTest to determine whether a drench is warranted.
  • A very high burden is required before you can discern weight loss or anaemia. At this stage the losses are much greater than the costs associated with regular testing, mustering and drenching. Drench urgently as you are already making a loss.

Regular worm egg counts allow you to choose the cost-effective time to drench based on drenching threshholds.

It is false economy to wait until obvious weight loss or anaemia is indicating that stock need drenching, as significant stock, wool and meat value is already being lost.

The worm egg count thresholds at which drenching should occur have been developed to ensure they are cost-effective and sustainable—unnecessary drenches should be avoided as they can contribute to drench resistance. They also include occasional strategic drenching at lower thresholds to manage worm contamination on specific paddocks, which gives a benefit across the property.

What should I drench with?

This perennial question has no single right answer because it depends on the drench resistance profile on your property.

Step 1: do a DrenchTest or DrenchCheck as soon as you can.

Step 2: use these three equally important principles when choosing a drench product:

1. Use drenches most effective on your property.

2. Use an effective combination of two or more drench groups, either in a multi-active product or using more than one product concurrently (up the race with one and then the other) to combine different drench groups. The higher the efficacy of each drench group and the more drench groups included in the combination, the greater the benefit for slowing drench resistance. 

3. Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year. 

If you don’t have a DrenchTest result less than 3 years old (where a number of drenches are tested), next time you drench, plan to do a DrenchCheck.